Lithography is conventionally used in the fabrication of semiconductor devices. In optical lithography, a photosensitive film, a photoresist, is patterned by a photomask. The photomask has areas composed of chrome, or some other opaque or partially transmitting material, in a pattern which corresponds to the desired circuitry for the device. The photomasked photoresist is exposed to light from a light source. After exposure, the photomask is removed, leaving a photoresist with the desired pattern. An etch or implantation of the device's substrate may be performed based upon the photoresist pattern.
FIG. 1 illustrates a conventional photomask 110 and photoresist 130 as used in lithography. The photomask 110 may comprise fused silica. On the bottom side of the photomask 110 are areas 120. The photomask 110 and photoresist 130 are treated with light from a light source (not shown). The light shines through the photomask 110 where there are no areas 120. The light is reflected or absorbed where there are areas 120. Some of the light that passes through the photomask 110 continues through a lens 140, which projects an image of the mask pattern onto the photoresist 130, which undergoes a chemical reaction when exposed to light. Portions of the photoresist 150 are exposed to the light while portions of the photoresist 160 are ideally not exposed to the light.
However, approximately 4-5% of the light is lost through reflections off each of the two surfaces of the photomask, as illustrated by arrows 170 in FIG. 1. When a large number of devices are fabricated on a substrate, this amount of loss is costly for the manufacturer. In addition, light reflected from the substrate can be transmitted back through the lens and back to the photomask 110. A portion of this light is then reflected by the photomask 110 back to the photoresist 160 on the wafer. Such reflected light is not part of the intended image of the photomask 110, and it can degrade the quality of the light pattern in the photoresist 160.
Accordingly, what is needed is a system and method for improving the transmission of light through photomasks. The method and system should decrease the loss of light due to reflections and decrease undesired exposure of portions of the photoresist. The present invention addresses such a need.